This invention relates to a circuit for detecting a vertical synchronizing signal from a television signal.
A television signal contains two kinds of synchronizing signal, i.e., a vertical synchronizing signal and a horizontal synchronizing signal. For separating the vertical synchronizing signal, a frequency separation circuit has been used in the past. This circuit, utilizing the fact that the vertical synchronizing signal contains a relatively low frequency component, separates the vertical synchronizing signal by passing a signal containing both vertical and horizontal synchronizing signals through a low-pass filter. Since, however, rise and fall of the output of the low-pass filter in this frequency separation circuit are not sufficiently clear, start and end of the vertical synchronizing signal cannot be detected so accurately by this circuit. Besides, in a case where a frequency component which is equivalent to a vertical synchronizing signal has been generated due to noise or dropout, such frequency component is likely to be mistaken for vertical synchronizing signal thereby leading to an erroneous detection.
It is, therefore, an object of the invention to provide a vertical synchronizing signal detection circuit capable of accurately detecting a vertical synchronizing signal from a television signal.